The Trainspotting star, a longtime UNICEF U.K. ambassador, has added his voice to the campaign to help victims of the disaster as floodwaters continue to rise.
In the video, which will air online and on British TV, McGregor says, "We've all seen the shocking pictures on the news about the floods in Pakistan. These floods have affected more people than the Haiti earthquake and 2004 tsunami combined and as in all disasters, it's the children who have been hardest hit - an estimated 3.5 million children in Pakistan are now in need of urgent help.
"I spoke to the deputy representative of UNICEF in Pakistan this week, and she told me that children's homes and schools have been washed away. They have no food or clean water. The roads and vital supplies have disappeared overnight.
"Pakistan has never seen a disaster of this scale before. The situation is desperate and as a UNICEF Ambassador, I am asking you to do what you can to help children."
More than 1,500 people have been killed and an estimated one billion are left stranded after monsoon rains caused the worst flooding in Pakistan for a century.